The invention generally relates to a tuned CATV filter, and, in particular, to a two-tiered notch filter that has filter sections disposed on opposite sides of an integrated circuit board to allow substantial reduction in the overall length of the filter. The invention also relates to an electronic filter having a filter housing formed from inner and outer housing segments mechanically coupled to one another.
It has long been a customary practice for subscription television communication systems, i.e., a system in which the reception of a television program signal (generally recoverable by a group of television receivers) is limited to specifically authorized receivers, to scramble or encode their television transmission signals to prevent unauthorized reception by individuals who have not paid a subscription fee. Conventional hard-wired community antenna systems (CATV), for example, are representative of a typical subscription television communication system, although other systems have been developed that employ radio frequency (RF) transmission to subscribers instead of hardwiring. If the subscription television system provides more than one channel, it is also desirable to provide selected scrambling or securing of individual channels that are used to provide premium services.
One of the methods employed to prevent unauthorized reception is to add a scrambling signal or interfering carrier signal to the television transmission signal between its aural and visual carriers. The scrambling signal is added to the television transmission signal to provide additional information which causes a television receiver to reconstitute the scrambled television transmission signal in an incoherent form. The scrambling signal is removed by passing the scrambled television signal through a tuned notch filter at the site of an authorized television receiver.
Tuned notch filters to be utilized in the video/audio frequency removal or descrambling process must meet several critical requirements. For example, it is critical that the tuned notch filter have a high degree of stability and reliability. It must have high attenuation over the desired frequency band and sharp skirts to minimize unwanted attenuation of adjacent channels. In addition to the electrical requirements, the tuned notch filter must be small, preferably less than 0.825 inch in diameter, to enable the notch filter to be utilized in typical installations where one or more filters are mounted on a directional tap on a strand, in a pedestal, or some other small enclosure. In today""s market, more premium services are being offered which necessitates connecting more than one tuned notch filter end to end. It is therefore desirable to minimize the length of the tuned notch filter to allow more notch filters to be connected in the available space of the mounting locations.
The industry has made great strides at reducing the length of tuned filters, such as shown in U.S. Pat. No. 5,770,983, the entirety of which is incorporated herein by reference. The ""983 patent discloses a reduced length tuned filter that has transverse oriented coils as shown, for example, in FIG. 3 thereof. This filter represents a significant improvement in reducing the length of a tuned filter. It would be desirable, however, to reduce the length of the filter still further.
In view of the above, it is the object of the invention to provide a tuned filter which is shorter in length, while maintaining the electrical performance and diameter requirements.
It is an object of the present invention to provide a tuned filter of reduced length when compared to the above-discussed prior art.
A reduced length tuned filter in accordance with one embodiment of the present invention includes a housing having a female connector end and an opposed male connector end, and a circuit board positioned within the housing and having a first end proximate the female connector end and an opposed second end proximate the male connector end. The circuit board has a first filter section positioned on an upper surface thereof, a second filter section positioned on a lower surface thereof, and a ground plane positioned between the upper and lower surfaces. The first filter section is in electrical communication with the second filter section, and each filter section includes at least one tunable coil. An input terminal is connected to the first end of the circuit board and extends within the female connector end of the housing, and an output terminal is connected to the second end of the circuit board and extends within the male connector end of the filter housing. A first isolation shield is provided in electrical communication with the ground plane, and extends from the upper surface of the circuit board in a direction substantially perpendicular thereto and proximate one of the first and second ends of the circuit board, and a second isolation shield is provided in electrical communication with the ground plane, and extends from the lower surface of the circuit board in a direction substantially perpendicular thereto and proximate the other one of the first and second ends of the circuit board.
The overall length of the filter in accordance with the present invention can be reduced substantially by positioning the first and second filter sections on opposite surfaces of the same circuit board. This two-tiered arrangement effectively allows the two filter sections, which are necessary to form a frequency notch, to be positioned within the footprint of a single filter section. Moreover, by positioning a ground plane between the opposed surfaces of the circuit board (e.g., within the center plane of the circuit board), the necessary and critical magnetic shielding between the two filter sections can be achieved to a substantial degree merely by the presence of the ground plane. The first and second isolation shields positioned on opposite surfaces and at opposed ends of the circuit board complete the necessary shielding between the two filter sections.
In accordance with a preferred embodiment of the present invention, the circuit board has opposed longitudinally extending side surfaces, and the ground plane is exposed along those side surfaces. This allows the side surfaces to be attached directly to the inner surface of the filter housing through soldering, for example, to maximize shielding between the two filter sections, especially at the side regions of the circuit board.
In accordance with another embodiment of the present invention, an electronic filter is provided that has a filter housing formed from inner and outer housing segments that are mechanically coupled together through first and second mechanical engagement members formed on the outer surface of the inner housing segment and the inner surface of the outer housing segment, respectively. It is preferred that the first and second mechanical engagement members are complementary in shape, one example being a spline and knurl arrangement.
In accordance with yet another embodiment of the present invention, the electronic filter is provided with inner and outer housing segments, and opposed longitudinal slots are formed in the inner surface of the inner housing segment. Opposite edges of the circuit board are then positioned within the longitudinal slots of the inner housing segment. This facilitates electrical contact between the housing and the circuit board, particularly if the circuit board includes an interior ground plane.
In accordance with another embodiment of the present invention, each of the tunable coils is a radial leaded coil, in that the tuning bore of each coil is positioned substantially parallel to the ground plane of the circuit board. Positioning the coils to extend parallel to the ground plane allows for a substantial reduction in the profile of the filter sections, which in turn allows the filter sections to be positioned on opposite sides of the circuit board and still fit within a filter housing not exceeding 0.825 inches in diameter.
It is also preferred to position the coils on the circuit board so as to extend in a transverse direction substantially perpendicular to the longitudinal axis of the filter housing. This arrangement allows the tuning bores of the coils to be accessed from tuning access holes positioned in the side of the filter housing. Accordingly, the coils can be fine tuned after the filter is substantially assembled, but before the final assembly step of positioning the filter housing within an outer protective metal jacket.